1. Field of the Invention
This invention relates to a method for providing a nuclear fuel and a fuel element provided with a nuclear fuel made by such a method.
2. Description of the Related Art
It is known to produce nuclear fuels containing relatively low densities of highly enriched uranium. Highly enriched uranium (HEU) has the disadvantage that for political reasons, the use of this material is undesirable, because it can not only be deployed for peaceful purposes but can also be used for less peaceful applications, as for perpetrating terrorist attacks and/or manufacturing atomic bombs. For this reason in recent years the use of low enriched uranium (LEU), which has a 235-U content of less than 20%, is strongly encouraged. In view of the above-mentioned disadvantage of HEU, a nuclear fuel preferably comprises uranium not highly enriched in the 235-U isotope. A disadvantage of such a fuel, however, is that the total uranium content (the sum of all isotopes) in an LEU-containing fuel element must be much higher than that in an HEU-containing fuel element, in order to obtain a similar reactivity. However, metallic uranium inherently has insufficient mechanical stability during irradiation to be used in elemental form.
It is known to impart stability to the uranium by using it in a chemical composition with silicon, e.g. U3Si2. In this composition, however, the density of the uranium still cannot always attain the desired higher value to be able to provide a reactivity sufficiently high to enhance the utility or the economics of the reactor. For this reason a new class of nuclear fuels for use in research, test and radioisotope-production reactors is being developed based on uranium-molybdenum alloys. The high uranium density of these alloys should make it possible to fuel these reactors using LEU instead of HEU, without causing a large decrease of the neutron flux in these reactors. These uranium-molybdenum alloys offer the further advantage of being able to be reprocessed in currently operating reprocessing plants.
Uranium-molybdenum alloys are currently being tested as dispersions of alloy fuel particles in a non-fissionable matrix material, such as aluminum, and as a monolithic fuel. Both types of fuel are enclosed in a cladding such as aluminum. U.S. Pat. No. 5,978,432 describes one method of producing a dispersion fuel using uranium-molybdenum alloys. The most common design of a Material Test Reactor (MTR) fuel element using U-Mo fuel and U-Mo fuel plates is as follows. U-Mo particles are dispersed into an aluminum matrix. The dispersed particles and the aluminum matrix make up a thin fuel layer, which is placed between two thin cladding plates of aluminum alloy. Such a cladding is, for example, described in the U.S. Pat. No. 4,963,317. In the present invention, the aluminum matrix is to be understood to comprise the aluminum of the fuel only. Therefore, the bulk of the aluminum cladding does not form part of the matrix in the present invention.
The thus-obtained fuel plates, each comprising a fuel layer enclosed by cladding plates, are either curved or flat. Approximately 20 of such plates form a fuel element together with structural components. Cooling water that flows between the plates cools the fuel plates fuelling a reactor during the operation thereof.
Although the use of uranium-molybdenum alloys makes it possible to increase the uranium density in the nuclear fuel for research and test reactors, the use of molybdenum carries the penalty of a significant neutron absorption, which decreases the effect of the added uranium, resulting in a relatively low reactivity. Therefore, the application of molybdenum in a nuclear fuel has been unfavourable albeit its advantages.